Solid state lighting is rapidly expanding its penetration, bringing to the market increased lighting efficiency, longer life and additional capabilities. One example of solid stage lighting is the use of light emitting diodes (LEDs), which are available in a plurality of colors. By combining the optical output of a plurality of colored LEDs a range of colors may be output. In one non-limiting example, the use of red, green and blue LEDs placed in proximity and behind a diffuser closes a complete range of colors by adjusting the relative intensity of the constituent LEDs, while the overall intensity of the constituent LEDs may be further adjusted to control the average overall luminance. Alternatively, LEDs producing a white output light are available, the white output typically being a result of a native blue or ultraviolet LED whose optical output excites a phosphor coating.
In order to economically control a large plurality of LEDs together producing sufficient light, the LEDs are typically supplied as a serially connected LED string, thereby sharing a single current. Each of the LED strings may be intensity controlled by one or both of amplitude modulation (AM), in which the value of the current through the LED string is adjusted, and pulse width modulation (PWM) in which the duty rate is controlled to adjust the average intensity over time. Thus, total intensity and color may be controlled by any combination of AM and PWM.
One of the challenges of solid state lighting, and LED lighting in particular, is to be directly compatible with current lighting fixtures and installations. Thus, in an ideal world, an incandescent bulb would be directly replaceable with a solid state lighting equivalent, without requiring a change in sockets, switches or dimmers. Certain solid state lighting solutions based on LED strings have been produced which fit into current lighting sockets, however the performance in cooperation with standard dimmer installations, which are typically thyristor based dimmers, have been less than satisfactory.